A typical large-scale offshore wind power plant (sometimes referred to as a wind farm) architecture consists of wind turbines, a medium voltage collection system, an offshore substation, a high voltage transmission system and an onshore substation to interface with a main power grid. For close-to-shore wind power plants, high voltage AC (HVAC) transmission systems are used. For offshore wind power plants with long distances to shore, voltage source converter based high voltage DC (VSC-HVDC) systems have proven technically advantageous and cost-effective over conventional HVAC solutions.
Currently, the collection grid associated with a windfarm utilizes medium voltage alternating current (AC) networks typically at 33 kV. In such a configuration, step-up transformers are needed at both the wind turbines and at the offshore platform substation. The wind turbine step-up transformers increase the AC output power voltage from 690 volts or 3.3 kV to 33 kV of the collection grid. Step-up transformers at the offshore platform increase the AC power voltage from 33 kV to a transmission voltage of 150-245 kV and then transfer the wind power onshore by HVAC submarine cables or to the voltage level matching a AC/DC converter of the HVDC transmission system.
In a desire to improve system efficiency from wind turbine generators to grid connection points, it is believed that DC connection strategies could be extended from high voltage DC to the wind turbine generator outputs. Such a configuration and the resulting DC connection system could potentially reduce the total cost of the power converters and improve the overall system efficiency and performance. As a result, several DC connection systems have been developed and they can be categorized into various configurations. The first configuration is a DC system with two-stage DC/DC power conversion wherein DC-DC converters are used at the wind turbines and the offshore platform. The second configuration is a DC system with one stage DC/DC power conversion, wherein DC-DC converters are positioned at the offshore platform. A third configuration is a DC system with one stage DC/DC power conversion, wherein DC-DC converters are used at the wind turbine generators. A final configuration is a DC system with one stage DC/DC power conversion wherein DC-DC converters are at the wind turbine generators in a series connection. In the first embodiment described above a primary DC-DC converter is located at each wind turbine and is connected to a DC collection network and a secondary DC-DC converter is positioned at the offshore platform and is connected between the DC collection network and the HVDC transmission line. Another configuration utilizes a DC transmission and collection system with DC outputs of multiple wind turbine converters, each comprising an AC-DC converter and a DC-DC converter, coupled in series to the DC transmission line.
Although the aforementioned embodiments are improvements in the art, it is believed that the DC connection concepts requiring two-stage DC-DC power conversions may not be competitive to the conventional solutions with a MVAC collection grid and HVDC transmission system considering overall system efficiency, cost and maintenance requirements. Moreover, the DC connection concepts with the series connection of wind turbines to reach the voltage level of HVDC transmission may not be feasible because of known technical issues. For example, such a DC connection concept with one stage distributed DC-DC conversion at the wind turbine may have limited applications because it is not practical to require HVDC insulation at wind turbines.
Therefore, there is a need in the art for a DC connection scheme with one stage centralized DC-DC power conversion. In particular, there is a need for a DC connection scheme that embodies a high range MVDC collection grid, in the range of 20 to 50 kV or higher, comprising multiple MVDC feeders and a MVDC bus bar system at the offshore platform. Indeed, there is a need for utilization of modular DC-DC converters for transferring high power from the MVDC collection system to a HVDC transmission system. In this regard, there are needs for control methods for operating modular DC-DC converters connected to the different MVDC bus sections to ensure balanced operation of the HVDC system.